Introduction

49
Table 9. The Mode of Action, Site of Uptake and Symptoms of Different Herbicide Groups, continued
Herbicide
Group
Mode of
Action
Site of
Uptake
Grass weeds
22 Contact Foliar Leaves wilted within hours,
desiccated in 1 to 3 days.
26 (grass
weeds
only)
Systemic
Immediate cessation of growth,
rapid desiccation of new leaves and
purpling and yellowing of older
tissues.
27 Systemic Foliar Some bleaching and whitening of
leaves.
How to Identify Weed Resistance
Weed symptoms/timing
Broadleaf weeds
Leaves wilted in 1 to 3 days, desiccated
and dead in 3 to 7 days.
See Group 4.
Leaves bleached and whitened in 2 to 10
days and death in 7 to 10 days.
It is important to avoid confusing herbicide failure caused by resistance with herbicide failure caused by various other
factors (such as weather or application errors). When a herbicide fails to control weeds because of weather or application
factors, that herbicide may work in the field the next season. But when herbicides fail because of the development of
resistance, they will fail in subsequent years, regardless of weather or application procedures.
Herbicide resistance should be suspected under the following conditions:
• A weed species that the herbicide controlled in previous seasons now escapes the treatment, while other weeds that
appear on the label continue to be controlled in the field.
• The escapes cannot be attributed to adverse weather or emergence after application (if a post-emergence product is
in question).
• Irregular-shaped patches of a weed develop where the herbicide gives little or no control.
• Records of the past history of the field show repeated use of the same herbicide, or combinations of herbicides, that
kill the weed in question in the same way.
Weed Control
Table 10. Herbicide-Resistant Weeds in Western Canada
Weed Herbicide Group Locations Confirmed
Canada Fleabane Group 9 Occurs in several US states
Cleavers Group 2 AB, MB, SK
Group 4
AB
Multiple combinations of: Groups 2 & 4 AB
Chickweed Group 2 AB, SK
Cow Cockle Group 2 AB
Green Foxtail Group 1 AB, MB, SK
Group 2
MB
Group 3
AB, MB, SK
Multiple combinations of: Groups 1 & 3 MB, SK
Hemp-nettle Group 2 AB, MB
Group 4
AB
Multiple combinations of: Groups 2 & 4 AB
Kochia Group 2 AB, MB, SK (overwhelming majority)
Group 4 (dicamba and fluroxypyr)
SK
Group 5
Occurs in North Dakota and Montana
Group 9 (glyphosate)
AB, MB, SK